Electronic location monitoring system

ABSTRACT

A method and apparatus for operating a monitoring unit operable to receive signals from a mobile transmitter in an electronic location monitoring system involves causing a first receiver to change a signal reception threshold thereof for receiving signals from the mobile transmitter, in response to proximity of the monitoring unit relative to a reference position whereby the receiver rejects signals received from the mobile transmitter that have a signal strength below the signal reception threshold and receives and processes signals received from the mobile transmitter that have a signal strength above the signal reception threshold. Alternatively, detection of a first monitoring unit in proximity to a second monitoring unit may cause the second monitoring unit to take over receiving signals from the mobile transmitter while the first monitoring unit is put into a low power mode.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Application No.60/497,017 filed on Aug. 22, 2003.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to remote monitoring and more particularly toremote monitoring in an electronic location monitoring system.

2. Description of Related Art

As an alternative to incarcerating offenders for minor infractions, moreand more judicial and law enforcement agencies prefer to place suchoffenders under house arrest and monitor their movement using electroniclocation monitoring equipment. As a result, there is less space taken inalready overcrowded detention facilities such as prisons, thuseliminating the cost of incarceration, while at the same time allowingoffenders to avoid prison and provide them with the opportunity toattend school or earn a wage and participate in counseling orrehabilitation programs.

Electronic location monitoring equipment usually involves the use of anidentification tag securely attached to the individual's body, whichperiodically transmits a unique code to an RF receiver device placed inthe incarcerated individual's place of confinement (usually his or herhouse). The receiver listens for transmissions from the identificationtag, and if a transmission is not received within a predetermined amountof time, an event is logged by the receiver indicating that theidentification tag, and hence the individual wearing it, is no longer inthe vicinity of the receiver. In turn, the receiver transmits a statusmessage to a monitoring station over a communication channel such as thePSTN (Public Switched Telephone Network) indicating the presence orabsence of the individual at the monitored site.

The electronic location monitoring approach described above works wellto ensure that the individual remains in the monitored area according tothe conditions of his/her parole. However, once the individual is awayfrom the monitored site and out of range of the receiver, the individualcannot be reached. Law enforcement agencies usually want to ensure thatthe individual is at particular locations (not just one monitored site)at particular times. For example the individual may be required to be atwork during certain hours, at a counseling workshop during other hours,and at home at all other times. In some cases, it is also necessary toensure that the individual stays away from certain locations (for e.g.,schools, parks, home of an ex-spouse, etc.).

To overcome this problem, systems are known in which an incarceratedindividual wears a transmitter and carries a receiver operable toreceive signals from the transmitter. The receiver may performcalculations based on position signals it may receive to determinewhether the individual is in an allowable location and may permit theindividual to venture outside of a single area. One problem with thistype of system however is that the individual must carry the receiverwith him/her at all times, which can be somewhat inconvenient. Forexample when the receiver is being charged, the individual must stayrelatively near the charger. Consequently, this type of system can berestrictive.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided amethod for operating a monitoring unit operable to receive signals froma mobile transmitter in an electronic location monitoring system. Themethod involves causing a receiver of the monitoring unit to change asignal reception threshold thereof for receiving signals from the mobiletransmitter, in response to proximity of the monitoring unit relative toa reference position whereby the receiver rejects signals received fromthe mobile transmitter that have a signal strength below the signalreception threshold and receives and processes signals received from themobile transmitter that have a signal strength above the signalreception threshold.

Changing the signal reception threshold may involve causing the receiverto decrease the signal reception threshold when the monitoring unit isin proximity to the reference position and causing the receiver toincrease the signal reception threshold when the monitoring unit is notin proximity to the reference position.

The method may involve detecting proximity of the monitoring unitrelative to the reference position.

The method may involve producing a proximity signal for reception by thereceiver, in response to detecting the change in proximity of themonitoring unit relative to the reference position, whereby the receiverchanges the signal reception threshold in response to the proximitysignal.

The method may involve causing the proximity signal to indicate themonitoring unit is in proximity to the reference position when themonitoring unit is coupled to a docking station.

The method may involve causing the proximity signal to indicate themonitoring unit is not in proximity to the reference position when themonitoring unit is uncoupled from a docking station.

The method may involve receiving position signals at the monitoringunit, the position signals representing a geographical position of themonitoring unit, and producing the proximity signal in response to theposition signals.

The method may involve causing the proximity signal to indicate themonitoring unit is in proximity to the reference position when theposition signals meet a first position criterion.

The first position criterion may be met when the position signalsindicate the monitoring unit is within a first range of the referenceposition.

The method may involve causing the proximity signal to indicate themonitoring unit is not in proximity to the reference position when theposition signals meet a second position criterion.

The second position criterion may be met when the position signalsindicate the monitoring unit is within a second range of the referenceposition.

In accordance with another aspect of the invention, there is provided anapparatus operable to receive signals from a mobile transmitter in anelectronic location monitoring system. The apparatus includes aproximity signal generator operable to generate a proximity signal inresponse to proximity of the apparatus relative to a reference position,a receiver operable to receive signals from the mobile transmitter, thereceiver having a signal reception threshold responsive to the proximitysignal, such that the receiver rejects signals from the mobiletransmitter that have a signal strength below the signal receptionthreshold and receives signals from the mobile transmitter that have asignal strength above the signal reception threshold and a processorcircuit operable to process signals received by the receiver to produceprocessed signals, the processor circuit being operable to communicatewith a transmitter to cause the transmitter to transmit the processedsignals to a monitoring station.

The receiver may be operable to decrease the signal reception thresholdwhen the proximity signal indicates the apparatus is in proximity to thereference position and the receiver may be operable to increase thesignal reception threshold when the proximity signal indicates apparatusis not in proximity to the reference position.

The proximity signal generator may comprise a detector operable todetect proximity of the apparatus to the reference position.

The detector may be operable to detect coupling of the apparatus to adocking station, and to cause the proximity signal to indicate theapparatus is in proximity to the reference position when the apparatusis coupled to the docking station.

The detector may be operable to detect uncoupling of the receiver fromthe docking station and to cause the proximity signal to indicate theapparatus is not in proximity to the reference position when theapparatus is uncoupled from the docking station.

The detector may comprise a position signal receiver operable to receiveposition signals representing a geographical position of the apparatus,the proximity signal generator being operable to produce the proximitysignal in response to the position signals.

The detector may be operable to cause the proximity signal to indicatethe apparatus is in proximity to the reference position when theposition signals meet a first position criterion.

The detector may be configured to determine the first position criterionis met when the position signals indicate the apparatus is within afirst range of the reference position.

The detector may be operable to cause the proximity signal to indicatethe apparatus is not in proximity to the reference position when theposition signals meet a second position criterion.

The detector may be configured to determine the second positioncriterion is met when the position signals indicate the apparatus iswithin a second range of the reference position.

The position signal receiver may comprise a global positioning system(GPS) signal receiver.

The position signal receiver may include a wireless receiver operable toreceive position signals from a wireless communication network.

The position signal receiver may comprise a message receiver operable toreceive messages from a messaging system.

The message receiver may be operable to receive a message compatiblewith the SMS message format or the GPRS/CDMA 1X, CDPD, Mobitex, or 1XRTTdata packet format, or any other suitable format.

The apparatus may be incorporated into a modular battery unit.

The apparatus may further include a first mobile wireless transceiverpowered by the modular battery unit and in communication with theapparatus such that the wireless transceiver is operable to transmit theprocessed signals to the monitoring station.

The processor circuit may be configured to implement a portion of theproximity signal generator.

The processor circuit may be configured to cause the processed signalsto include a representation of the proximity signal.

In accordance with another aspect of the invention, there is provided anapparatus operable to receive signals from a mobile transmitter in anelectronic location monitoring system. The apparatus includes provisionsfor generating a proximity signal in response to proximity of theapparatus relative to a reference position, provisions for receivingsignals from the mobile transmitter, the provisions for receiving havinga signal reception threshold responsive to the proximity signal, suchthat signals from the mobile transmitter that have a signal strengthbelow the signal reception threshold are rejected and such that signalsfrom the mobile transmitter that have a signal strength above the signalreception threshold are received and provisions for processing signalsreceived by the receiver to produce processed signals, the provisionsfor processing being operable to communicate with a transmitter to causethe transmitter to transmit the processed signals to a monitoringstation.

The provisions for receiving may be operable to decrease the signalreception threshold when the proximity signal indicates the apparatus isin proximity to the reference position and operable to increase thesignal reception threshold when the proximity signal indicates apparatusis not in proximity to the reference position.

In accordance with another aspect of the invention, there is provided amethod for switching monitoring units in an electronic locationmonitoring system comprising a mobile transmitter operable to transmitto at least one of first and second monitoring units operable tocommunicate with a common monitoring station. The method involvesdetecting proximity of the first monitoring unit relative to the secondmonitoring unit, producing a proximity signal in response to detectingproximity of the first monitoring unit relative to the second monitoringunit, for reception by the first monitoring unit to cause the firstmonitoring unit to enter a receive mode in which processing of signalsreceived from the mobile transmitter by the first monitoring unit ispermitted when the first monitoring unit is not in proximity to thesecond monitoring unit or a low power mode in which processing ofsignals received from the mobile transmitter by the first monitoringunit is prevented when the first monitoring unit is in proximity to thesecond monitoring unit and causing the second monitoring unit to receiveand process signals from the mobile transmitter when the proximitysignal indicates the first monitoring unit is in proximity to the secondmonitoring unit and causing the second monitoring unit to ceasereceiving signals from the mobile transmitter when the proximity signalindicates the first monitoring unit is not in proximity to the secondmonitoring unit.

The method may involve causing the proximity signal to indicate thefirst monitoring unit is in proximity to the second monitoring unit whenthe first monitoring unit is coupled to a docking station associatedwith the second monitoring unit.

The method may involve causing the proximity signal to indicate thefirst monitoring unit is not in proximity to the second monitoring unitwhen the first monitoring unit is uncoupled from a docking stationassociated with the second monitoring unit.

Detecting may involve measuring, near the second monitoring unit, signalstrength of signals received from the mobile transmitter, and whereinthe proximity signal is produced in response to measured signalstrength.

The method may involve causing the proximity signal to indicate thefirst monitoring unit is in proximity to the second monitoring unit whenmeasured signal strength meets a first signal strength criterion.

The first signal strength criterion may be met when the measured signalstrength is above a first signal strength threshold value.

The method may involve causing the proximity signal to indicate thefirst monitoring unit is not in proximity to the second monitoring unitwhen measured signal strength meets a second signal strength criterion.

The second signal strength criterion may be met when the measured signalstrength is below a second signal strength threshold value.

The first and second signal strength threshold values may be the same.

Detecting may comprise receiving position signals at the firstmonitoring unit, the position signals representing a geographicalposition of the first monitoring unit, and producing the proximitysignal in response to the position signals.

The method may involve causing the proximity signal to indicate thefirst monitoring unit is in proximity to the second monitoring unit whenthe position signals meet a first position criterion.

The first position criterion may be met when the position signalsindicate the first monitoring unit is within a first range of areference position.

The method may involve causing the proximity signal to indicate thefirst monitoring unit is not in proximity to the second monitoring unitwhen the position signals meet a second position criterion.

The second position criterion may be met when the position signalsindicate the first monitoring unit is within a second range of thereference position.

The reference position may represent an approximate geographicalposition of the second monitoring unit.

Producing the proximity signal may comprise causing a message to betransmitted to the first monitoring unit.

The method may involve causing a message to be transmitted to the firstmonitoring unit through a messaging service.

Causing a message to be transmitted to the first monitoring unit througha messaging service may involve causing a message compatible with theSMS message format or the GPRS/CDMA 1X, CDPD, Mobitex, or 1XRTT datapacket format, or any other suitable format to be transmitted to thefirst monitoring unit.

The message may include a power down command when the first monitoringunit is in proximity to the second monitoring unit and the message mayinclude a power up command when the first monitoring unit is not inproximity to the second monitoring unit.

In accordance with another aspect of the invention, there is provided anapparatus for switching monitoring units in an electronic locationmonitoring system comprising a mobile transmitter operable to transmitto at least one of first and second monitoring units operable to processsignals received from the mobile transmitter and transmit processedsignals to a monitoring station. The apparatus includes a detectoroperable to produce a proximity signal indicating proximity of the firstmonitoring unit relative to the second monitoring unit and a firsttransmitter operable to cause the proximity signal to be communicated tothe first monitoring unit such that, when the proximity signal indicatesthe first monitoring unit is in proximity to the second monitoring unitthe first monitoring unit enters a low power mode in which signals arenot received from the mobile transmitter by the first monitoring unitand when the proximity signal indicates that the first monitoring unitis not in proximity to the second monitoring unit the first monitoringunit enters a receive mode in which signals from the mobile transmitterare received and processed by the first monitoring unit. The apparatusfurther includes a second transmitter operable to cause the proximitysignal to be transmitted to the second monitoring unit such that inresponse to the proximity signal the second monitoring unit receives andprocesses signals from the mobile transmitter when the proximity signalindicates the first monitoring unit is in proximity to the secondmonitoring unit and ceases receiving signals from the mobile transmitterwhen the proximity signal indicates the first monitoring unit is not inproximity to the second monitoring unit.

The detector may be operable to cause the proximity signal to indicatethe first monitoring unit is in proximity to the second monitoring unitwhen the first monitoring unit is coupled to a docking station.

The detector may be operable to cause the proximity signal to indicatethe first monitoring unit is not in proximity to the second monitoringunit when the first monitoring unit is uncoupled from a docking station.

The detector may comprise a signal strength detector operable to detectsignal strength of signals received from the mobile transmitter, and thedetector may produce the proximity signal in response to measured signalstrength.

The detector may cause the proximity signal to indicate the firstmonitoring unit is in proximity to the second monitoring unit whenmeasured signal strength meets a first signal strength criterion.

The detector may determine the first signal strength criterion is metwhen the measured signal strength is above a first signal strengththreshold value.

The detector may cause the proximity signal to indicate the firstmonitoring unit is not in proximity to the second monitoring unit whenmeasured signal strength meets a second signal strength criterion.

The detector may determine the second signal strength criterion is metwhen the measured signal strength is below a second signal strengththreshold value.

The first and second signal strength threshold values may be the same.

The detector may include a position signal receiver operable to receiveposition signals representing a geographical position of the firstmonitoring unit, and the detector may produce the proximity signal inresponse to the position signals.

The detector may cause the proximity signal to indicate the firstmonitoring unit is in proximity to the second monitoring unit when theposition signals meet a first position criterion.

The detector may determine the first position criterion is met when theposition signals indicate the first monitoring unit is within a firstrange of a reference position.

The detector may cause the proximity signal to indicate the firstmonitoring unit is not in proximity to the second monitoring unit whenthe position signals meet a second position criterion.

The detector may determine the second position criterion is met when theposition signals indicate the first monitoring unit is within a secondrange of the reference position.

The reference position may represent an approximate geographicalposition of the second monitoring unit.

The first transmitter may be operable to transmit a message representingthe proximity signal to the first monitoring unit.

The apparatus may further include a first messaging service interfaceoperable to transmit a message to the first monitoring unit through amessaging service.

The first messaging service interface may be operable to transmit amessage compatible with the SMS message format or the GPRS/CDMA 1X,CDPD, Mobitex, or 1XRTT data packet format, or any other suitable formatto be transmitted to the first monitoring unit.

The second transmitter may be operable to transmit a messagerepresenting the proximity signal to the second monitoring unit.

The apparatus may further include a second messaging service interfaceoperable to transmit a message to the second monitoring unit through amessaging service.

The second messaging service interface may be operable to transmit amessage compatible with the SMS message format or the GPRS/CDMA 1X,CDPD, Mobitex, or 1XRTT data packet format, or any other suitable formatto be transmitted to the second monitoring unit.

The apparatus may further include a modular battery unit, operable topower the first monitoring unit. The detector, the first transmitter andthe second transmitter may be incorporated into the modular batteryunit.

The apparatus may further include a housing and the detector, the firsttransmitter, the second transmitter and the second monitoring unit maybe housed in the housing.

The housing may have a docking station to facilitate docking of thefirst monitoring unit.

In accordance with another aspect of the invention, there is provided anapparatus for use in an electronic location monitoring system. Theapparatus includes a first monitoring unit comprising a first receiveroperable to receive signals from a mobile transmitter on a person to bemonitored, the first receiver having a low power mode in which signalsare not received from the mobile transmitter and a receive mode in whichsignals from the mobile transmitter are received. The first monitoringunit further includes a processor circuit for processing signalsreceived by the first receiver to produce processed signals and atransmitter operable to transmit the processed signals to a monitoringstation. The apparatus further includes a signaling unit comprising adetector operable to produce a proximity signal indicating proximity ofthe first monitoring unit relative to a second monitoring unit operableto receive, process and transmit signals to the monitoring station, thefirst receiver being responsive to the proximity signal such that, whenthe proximity signal indicates the first monitoring unit is in proximityto the second monitoring unit the first receiver is placed in the lowpower mode and when the proximity signal indicates that the firstmonitoring unit is not in proximity to the second monitoring unit thefirst receiver enters the receive mode. The signaling unit furtherincludes a proximity signal transmitter operable to cause the proximitysignal to be transmitted to the second monitoring unit, whereby inresponse to the proximity signal the second monitoring unit receives andprocesses signals from the mobile transmitter when the proximity signalindicates the first monitoring unit is in proximity to the secondmonitoring unit and ceases receiving signals from the mobile transmitterwhen the proximity signal indicates the first monitoring unit is not inproximity to the second monitoring unit. The first monitoring unitreceives and processes signals from the mobile transmitter and thesecond monitoring unit does not receive and process signals from themobile transmitter when the first monitoring unit is not in proximity tothe second monitoring unit and the second monitoring unit receives andprocesses signals from the mobile transmitter and the first monitoringunit does not receive and process signals from the mobile transmitterwhen the first monitoring unit is in proximity to the second monitoringunit.

The apparatus may further include a modular battery unit operable topower the first monitoring unit and the signaling unit and wherein thefirst monitoring unit and the signaling unit may be housed in themodular battery unit.

The apparatus may further include a portable wireless communicationappliance. The apparatus may be incorporated into the portable wirelesscommunication appliance.

The portable wireless communication appliance may include a cellulartelephone.

The detector may be operable to cause the proximity signal to indicatethe first monitoring unit is in proximity to the second monitoring unitwhen the first monitoring unit is coupled to a docking station.

The detector may be operable to cause the proximity signal to indicatethe first monitoring unit is not in proximity to the second monitoringunit when the first monitoring unit is uncoupled from a docking station.

The detector may comprise a signal strength detector operable to detectsignal strength of signals received from the mobile transmitter, and thedetector may produce the proximity signal in response to measured signalstrength.

The detector may cause the proximity signal to indicate the firstmonitoring unit is in proximity to the second monitoring unit whenmeasured signal strength meets a first signal strength criterion.

The detector may determine the first signal strength criterion is metwhen the measured signal strength is above a first signal strengththreshold value.

The detector may cause the proximity signal to indicate the firstmonitoring unit is not in proximity to the second monitoring unit whenmeasured signal strength meets a second signal strength criterion.

The detector may determine the second signal strength criterion is metwhen the measured signal strength is below a second signal strengththreshold value.

The first and second signal strength threshold values may be the same.

The detector may comprise a position signal receiver operable to receiveposition signals representing a geographical position of the firstmonitoring unit and the detector may produce the proximity signal inresponse to the position signals.

The detector may cause the proximity signal to indicate the firstmonitoring unit is in proximity to the second monitoring unit when theposition signals meet a first position criterion.

The detector may determine the first position criterion is met when theposition signals indicate the first monitoring unit is within a firstrange of a reference position.

The detector may cause the proximity signal to indicate the firstmonitoring unit is not in proximity to the second monitoring unit whenthe position signals meet a second position criterion.

The detector may determine the second position criterion is met when theposition signals indicate the first monitoring unit is within a secondrange of the reference position.

The reference position may represent an approximate geographicalposition of the second monitoring unit.

The proximity signal transmitter may be operable to transmit a messagerepresenting the proximity signal to the second monitoring unit.

The apparatus may further include a messaging service interface operableto transmit a message to the second monitoring unit through a messagingservice.

The messaging service interface may be operable to transmit a messagecompatible with the SMS message format or the GPRS/CDMA 1X, CDPD,Mobitex, or 1XRTT data packet format, or any other suitable format to betransmitted to the second monitoring unit.

In accordance with another aspect of the invention, there is provided adocking apparatus for a first monitoring unit in an electronic locationmonitoring system in which the first monitoring unit has a firstreceiver operable to receive signals from a mobile transmitter on aperson to be monitored and wherein the first monitoring unit is operableto process the signals to produce processed signals and to transmit theprocessed signals to a monitoring station. The docking apparatuscomprises a second monitoring unit comprising a second receiver operableto receive signals from the mobile transmitter, the second receiverhaving a non-receive mode in which signals are not received from themobile transmitter and a receive mode in which signals from the mobiletransmitter are received. The second monitoring unit includes aprocessor circuit for processing signals received by the second receiverto produce processed signals and a transmitter operable to transmit theprocessed signals to the monitoring station. The docking apparatusfurther includes a signaling unit comprising a detector operable toproduce a proximity signal indicating proximity of the first monitoringunit relative to the second monitoring unit, the second receiver beingresponsive to the proximity signal such that, when the proximity signalindicates the first monitoring unit is in proximity to the secondmonitoring unit the second receiver is placed in the receive mode andwhen the proximity signal indicates that the first monitoring unit isnot in proximity to the second receiver unit is placed in thenon-receive mode. The signaling unit further includes a proximity signaltransmitter operable to cause the proximity signal to be transmitted tothe first monitoring unit, whereby in response to the proximity signalthe first monitoring unit receives and processes signals from the mobiletransmitter when the proximity signal indicates the first monitoringunit is not in proximity to the second monitoring unit and ceasesreceiving signals from the mobile transmitter when the proximity signalindicates the first monitoring unit is in proximity to the secondmonitoring unit. The first monitoring unit receives and processessignals from the mobile transmitter and the second monitoring unit doesnot receive and process signals from the mobile transmitter when thefirst monitoring unit is not in proximity to the second monitoring unitand the second monitoring unit receives and processes signals from themobile transmitter and the first monitoring unit does not receive andprocess signals from the mobile transmitter when the first monitoringunit is in proximity to the second monitoring unit.

The docking station apparatus may include a docking port to which thefirst monitoring unit may be coupled.

The detector may be operable to cause the proximity signal to indicatethe first monitoring unit is in proximity to the second monitoring unitwhen the first monitoring unit is coupled to the docking station.

The detector may be operable to cause the proximity signal to indicatethe first monitoring unit is not in proximity to the second monitoringunit when the first monitoring unit is uncoupled from the dockingstation.

The detector may comprise a signal strength detector operable to detectsignal strength of signals received from the mobile transmitter and thedetector may produce the proximity signal in response to measured signalstrength.

The detector may cause the proximity signal to indicate the firstmonitoring unit is in proximity to the second monitoring unit whenmeasured signal strength meets a first signal strength criterion.

The detector may determine the first signal strength criterion is metwhen the measured signal strength is above a first signal strengththreshold value.

The detector may cause the proximity signal to indicate the firstmonitoring unit is not in proximity to the second monitoring unit whenmeasured signal strength meets a second signal strength criterion.

The detector may determine the second signal strength criterion is metwhen the measured signal strength is below a second signal strengththreshold value.

The first and second signal strength threshold values may be the same.

The detector may comprise a position signal receiver operable to receiveposition signals representing a geographical position of the firstmonitoring unit and the detector may produce the proximity signal inresponse to the position signals.

The detector may cause the proximity signal to indicate the firstmonitoring unit is in proximity to the second monitoring unit when theposition signals meet a first position criterion.

The detector may determine the first position criterion is met when theposition signals indicate the first monitoring unit is within a firstrange of a reference position.

The detector may cause the proximity signal to indicate the firstmonitoring unit is not in proximity to the second monitoring unit whenthe position signals meet a second position criterion.

The detector may determine the second position criterion is met when theposition signals indicate the first monitoring unit is within a secondrange of the reference position.

The reference position may represent an approximate geographicalposition of the second monitoring unit.

The first transmitter may be operable to transmit a message representingthe proximity signal to the first monitoring unit.

The apparatus may further include a messaging service interface operableto transmit a message to the first monitoring unit through a messagingservice.

The messaging service interface may be operable to transmit a messagecompatible with the SMS message format or the GPRS/CDMA 1X, CDPD,Mobitex, or 1XRTT data packet format, or any other suitable format to betransmitted to the first monitoring unit.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate embodiments of the invention,

FIG. 1 is a schematic diagram of an electronic location monitoringsystem according to a first embodiment of the invention,

FIG. 2 is a block diagram of a wireless communication applianceaccording to a first embodiment of the invention;

FIG. 3 is a schematic diagram of an electronic location monitoringsystem according to a second embodiment of the invention,

FIG. 4 is a block diagram of a signaling apparatus shown in FIG. 3;

FIG. 5 is a block diagram of a mobile communication applianceincorporating a monitoring unit and the signaling apparatus of FIG. 4;

FIG. 6 is a block diagram of a stationary communication applianceincorporating a monitoring unit and the signaling apparatus of FIG. 4;

DETAILED DESCRIPTION

Referring to FIG. 1, an electronic location monitoring system accordingto a first embodiment of the invention is shown generally at 10. Thesystem includes a mobile transmitter 12 which may be strapped onto anincarcerated person by non-removable bands, a strap, or other permanentsecuring means. The mobile transmitter may incorporate tamper detectionmechanisms that, where a strap is used, may detect severing of a strap,or a change in the resistance of a strap circuit, caused by attemptingto electrically jumper it. The mobile transmitter may be housed in awaterproof casing and may also include tamper detection circuitry, whichmay include detection of case tampering or proximity to skin, forexample.

The mobile transmitter 12 periodically transmits a beacon signal 14encoded with information that may include a transmitter identificationcode, battery status code, tamper status code and/or other information.The beacon signal 14 may be transmitted periodically at a frequency ofabout 433 MHz, for example, but any radio frequency may be used.Furthermore, the mobile transmitter 12 may communicate with the receiveron multiple communication channels, so that if one channel experiencesinterference, the other communication channel may still be functional.The mobile transmitter 12 need only transmit the beacon signal 14 at apower level sufficient to permit the signal to be detected a shortdistance away. A power level sufficient to permit the beacon signal 14to be detected about 300 feet away is sufficient.

In the embodiment shown, the system 10 further includes a wirelesscommunication appliance 16, which may include a wireless telephone, forexample. Referring to FIG. 2, the wireless communication appliance 16includes a first transceiver 18 operable to receive from and transmitsignals to a base station 20 shown in FIG. 1 of a public communicationsnetwork such as a cellular telephone network, for communication to amonitoring station 22 via the same network or the public switchedtelephone network.

Referring back to FIG. 2, in the embodiment shown, the wirelesscommunication appliance 16 is powered by a removable modular batteryunit 24 having a bus interface 26 that allows it to communicate with aprocessor circuit 28 of the wireless communication appliance. Accordingto an aspect of the invention, a monitoring unit apparatus 30 operableto receive signals from the mobile transmitter 12 shown in FIG. 1 isincluded within the modular battery unit 24 and is operable tocommunicate with the processor circuit 28 of the wireless communicationappliance 16 through the bus interface 26. Alternatively, the apparatus30 may be incorporated as a functional block of the wirelesscommunication appliance 16, for example.

In the embodiment shown, the apparatus 30 includes a proximity signalgenerator 32 operable to generate a proximity signal in response toproximity of the monitoring unit apparatus 30 relative to a referenceposition, a receiver 34 operable to receive signals from the mobiletransmitter (12), and a processor circuit 36 operable to process signalsreceived by the receiver to produce processed signals for communicationto the monitoring station. The processor circuit 36 is operable tocommunicate with the first transceiver 18 of the wireless communicationappliance 16 through the bus interface 26 and the processor circuit 28of the wireless communication appliance to cause the first transceiver18 to transmit the processed signals to the monitoring station 22 shownin FIG. 1.

Of particular importance to the monitoring unit apparatus 30 is thereceiver 34 as it is specially configured to have a signal receptionthreshold responsive to the proximity signal, such that the receiverrejects signals from the mobile transmitter (12) that have a signalstrength below the signal reception threshold and receives signals fromthe mobile transmitter that have a signal strength above the signalreception threshold. In general, the receiver 34 is operable to decreasethe signal reception threshold (i.e., increase sensitivity) when theproximity signal indicates the monitoring unit apparatus 30 is inproximity to the reference position and to increase the signal receptionthreshold (i.e., decrease sensitivity) when the proximity signalindicates monitoring unit apparatus 30 is not in proximity to thereference position.

The proximity signal generator 32 includes a detector 38 operable todetect proximity of the monitoring unit apparatus 30 to the referenceposition. Various types of detectors may be employed for this purpose.For example, in one embodiment the detector 38 is operable to detectcoupling of the wireless communication appliance 16 to a docking station40 such as shown in FIG. 1, and to cause the proximity signal toindicate the monitoring unit apparatus 30 is in proximity to thereference position when the wireless communication appliance 16 iscoupled to the docking station. The position of the docking station isthus the reference position. Similarly, the detector 38 is operable todetect uncoupling of the wireless communication appliance from thedocking station 40 and to cause the proximity signal to indicate themonitoring unit apparatus 30 is not in proximity to the referenceposition when the wireless communication appliance 16 is uncoupled fromthe docking station.

Coupling to the docking station 40 may involve receiving the wirelesscommunication appliance 16 in a receptacle of the docking station,plugging the wireless communication appliance into a pigtail connectorof the docking station, or engaging a wireless link such as a Bluetooth®link, or infrared link between the docking station and the mobilewireless communication appliance, for example.

A detector operable to detect coupling and uncoupling may be implementedby causing the processor circuit 36 in the apparatus to communicatethrough the bus interface 26 with the processor circuit 28 of thewireless communication appliance 16 to receive a signal indicating thatthe communication appliance has been docked in a docking stationcompatible with the communication appliance. The processor circuit 28 inthe wireless communication appliance may detect whether the wirelesscommunication appliance 16 is placed in the docking station. Amodification may be made to a program controlling the processor circuit28 in the wireless communication appliance 16 to cause it to set a flagat a port accessible through the bus interface 26 and readable by theprocessor circuit 36 of the apparatus 30, depending upon whether thewireless communication appliance 16 is or is not in the docking station(40). The flag itself may act as the proximity signal, in which case theprocessor circuit 28 of the wireless communication appliance 16 may beconsidered to act as at least a portion of the proximity signalgenerator 32.

Alternatively, especially where the docking station 40 is configured tocharge the battery unit when the communication appliance is in thedocking station 40, the detector 38 may be configured to monitor signalsapplied to a battery 42 thereof and detect the direction of current flowrelative to the battery whereby the proximity signal is set active toindicate proximity of the monitoring unit apparatus 30 to the referenceposition when the current flow is into the battery and is set inactiveto indicate the wireless communication appliance is out of proximitywith the reference position, when the current flow is out of thebattery. In this embodiment, the reference position corresponds to theposition of the docking station (40).

In another embodiment, the detector 38 may include a position signalreceiver 44 operable to receive position signals representing ageographical position of the wireless communication appliance 16 andhence the monitoring unit apparatus 30 and the proximity signalgenerator 32 produces the proximity signal in response to the positionsignals. The position signal receiver 44 may be of a type that comprisesa global positioning system (GPS) signal receiver 46, or a Long RangeRadio Navigation (LORAN) receiver 48, for example. The position signalsproduced by the position signal receiver may include all or a subset ofthe following data items, for example: latitude, longitude, altitude,horizontal velocity, vertical velocity, heading, and a measure ofprecision of the location fix. The position information may betime-stamped and data logged along with the mobile transmitter 12 statusdata in a data log. The date and time used to time-stamp the data may beobtained from the GPS receiver or the wireless telecommunicationnetwork, or even possibly via messages from the central monitoringcenter computer that the wireless communication appliance reports to.These messages may include a message compatible with the SMS messageformat or the GPRS/CDMA 1X, CDPD, Mobitex, or 1XRTT data packet format,or any other suitable format, for example.

Alternatively, the position signal receiver may include the processorcircuit 36 of the apparatus 30, which may communicate with the processorcircuit 28 of the wireless communication appliance 16 to cause the firstreceiver to receive position signals provided by the wirelesscommunication network. Such signals may be produced by the wirelesscommunication network in response to triangulation techniques involvinga plurality base stations (20) in communication with the wirelesscommunication appliance 16, for example, as is known in the art. In suchan embodiment, the position signal receiver 44 may be considered toinclude the first transceiver 18 of the wireless communicationappliance. In one embodiment, the position signal receiver 44 mayinclude a message receiver 50 operable to receive messages from amessaging system, the messages including data representing positionsignals specifying the geographical position of the monitoring unitapparatus 30. Effectively, the position signal produced by the wirelessnetwork by triangulation, may be formatted in a message compatible withthe SMS message format or the GPRS/CDMA 1X, CDPD, Mobitex, or 1XRTT datapacket format, or any other suitable format, for example, and sent tothe wireless communication appliance, where position information may beextracted from the messages by the processor circuit 28 of the wirelesscommunication appliance or the processor circuit 36 of the apparatus 30.

In embodiments employing the position signal receiver 44, regardless ofhow the position signals are received at the apparatus, the detector 38is operable to cause the proximity signal to indicate the monitoringunit apparatus 30 is in proximity to the reference position when theposition signals meet a first position criterion. The detector 38 may beimplemented by the processor circuit 28 of the wireless communicationappliance 16 or by the processor circuit 36 of the apparatus 30, or byparts of these circuits or by both. The detector 38 may be configured todetermine the first position criterion is met when the position signalsindicate the monitoring unit apparatus 30 is within a first range of thereference position. For example, the detector 38 may be configured tocause the proximity signal to indicate the monitoring unit apparatus 30is in proximity to the reference position when the position signalsindicate the first receiver is within 300 feet (100 m) of the referenceposition. The reference position may correspond to the geographicalposition of the home, school or workplace of the incarceratedindividual, for example.

Similarly, the detector 38 may be operable to cause the proximity signalto indicate the monitoring unit apparatus 30 is not in proximity to thereference position when the position signals meet a second positioncriterion and the detector may be configured to determine the secondposition criterion is met when the position signals indicate themonitoring unit apparatus 30 is within a second range of the referenceposition. For example, the detector 38 may be configured to cause theproximity signal to indicate the monitoring unit apparatus 30 is not inproximity to the reference position when the position signals indicatethe apparatus is more than 300 feet (100 m) from the reference position.

When the monitoring unit apparatus 30 is not in proximity to thereference position, as indicated by the proximity signal, in response tothe proximity signal, the receiver 34 maintains the receive threshold ata relatively high value. The receive threshold can be held at arelatively high value because it is expected that the incarceratedindividual will carry the wireless communication appliance with him/herwhile away from his/her home or workplace, where the docking station 40may be kept, for example. Since the incarcerated individual wears themobile transmitter 12 and the wireless communication appliance 16 iscarried by the incarcerated individual when away from the dockingstation 40, the mobile transmitter and the receiver 34 of the apparatus30 are relatively close to each other and thus, the receiver need not beultra sensitive to the beacon signal.

When the monitoring unit apparatus 30 is in proximity to the referenceposition, as indicated by the proximity signal, in response to theproximity signal, the receiver 34 decreases its receive threshold to arelatively low value. If the reference position is the geographicallocation of the docking station, for example, the receive threshold isdecreased when the wireless communication appliance is in the dockingstation 40 in the home, school or workplace. The decrease in receivethreshold permits the incarcerated individual wearing the mobiletransmitter 12 to move freely within a radius of the docking station 40without having to carry the wireless communication appliance 16. Thewireless communication appliance 16 is thus less obtrusive.

Regardless of the level of the receive threshold, beacon signals havinga signal strength above the receive threshold are received and processedby the processor circuit 36 to extract the information pertaining to theidentification, tamper status and battery level. Such processing mayinvolve time-stamping a received beacon signal and storing it forproducing signals for transmission to the monitoring station 22 throughthe first transceiver 18 of the wireless communication appliance 16. Inaddition, a representation of the proximity signal may be included inthe signals transmitted to the monitoring station 22. The signalstransmitted to the monitoring station 22 may include signalsrepresenting a message such as a message compatible with the SMS messageformat or the GPRS/CDMA 1X, CDPD, Mobitex, or 1XRTT data packet format,or any other suitable format, for example. The monitoring station isthus informed of the status of the mobile transmitter 12.

When the monitoring unit apparatus 30 is not in proximity to thereference position, the receiver 34 may listen for transmissions fromthe mobile transmitter 12 having a signal strength associated with arange of about 10-20 feet. When the monitoring unit apparatus 30 is inproximity to the reference position, the receiver 34 may listen fortransmissions from the mobile transmitter 12 having a signal strengthassociated with a range of about 300 feet.

Regardless of the range, if the receiver 34 does not receive anytransmission from the mobile transmitter 12 within a specified duration,it may assume that the mobile transmitter 12 (and hence the individualwearing the transmitter device) is not at the location of the wirelesshandset. The processing by the processor circuit 36 may involve keepingtrack of the current status, i.e., whether the apparatus 30 is withinthe specified range of the mobile transmitter 12 in an internal datalog.

The position information as well as status information regarding themobile transmitter 12 identification may be transmitted to themonitoring station 22 using the wireless telecommunications network. Theposition information and status information may be transmittedcontinuously at specified intervals, or alternatively, the processorcircuit 36 may periodically store the position information in internaldata memory, and periodically send all location stored points at onetime to the monitoring station 22.

Similarly, it may also be possible for the monitoring station 22 to usemessaging to query the apparatus 30 for its current location as well asthe status of its matched mobile transmitter 12. An authorized user mayalso make such a query by messaging with a suitable wirelesscommunication device from any place that allows wireless communicationto be established. In addition to querying the current location andstatus from the wireless handset device, position and mobile transmitterstatus data logged in the internal memory of the apparatus may beselectively downloaded by messaging.

The monitoring station may also send messages to the apparatus to causeit to alter its receiver sensitivity, set criteria for determiningposition conditions, configure the receiver 34 for use with differentmobile transmitters 12, configure time intervals for transmissions tothe monitoring station, download alarm events, receive alert or warningmessages for annunciation to the incarcerated individual, and performother functions as may be required. Such messages may include a messagecompatible with the SMS message format or the GPRS/CDMA 1X, CDPD,Mobitex, or 1XRTT data packet format, or any other suitable format, forexample.

It will be appreciated that the apparatus 30 may include a receiver 34operable to receive different beacon signals from a plurality ofdifferent mobile transmitters 12, on a plurality of different channelsand that the channels may be scanned, for example for active beaconsignals: Each signal received on each channel may be provided to theprocessor circuit 36 by multiplexing, for example to consider eachsignal separately. This allows a single wireless communication applianceto be used to simultaneously monitor a plurality of incarceratedindividuals.

Referring to FIG. 3, a signaling apparatus facilitating switching ofmonitoring units in an electronic location monitoring system 100comprising a mobile transmitter 112 operable to transmit to at least oneof first and second monitoring units 114 and 116 operable to communicatewith a monitoring station 118 is shown generally at 101.

The first monitoring unit 114 includes a first receiver 113 (ortransceiver) operable to receive signals from the mobile transmitter112. The first receiver 113 has a low power mode in which signals arenot received from the mobile transmitter 112 and a receive mode in whichsignals are received from the mobile transmitter. The first monitoringunit 114 further includes a processor circuit 115 for processing signalsreceived by the first receiver 113 to produce processed signals andincludes a transmitter 117 (or transceiver) operable to transmitprocessed signals to the monitoring station 118. The first monitoringunit may be included within a wireless communication appliance 130, forexample.

The second monitoring unit 116 includes a second receiver 119 (ortransceiver) operable to receive signals from the mobile transmitter112. The second receiver 119 has a non-receive mode in which signals arenot received from the mobile transmitter 112 and a receive mode in whichsignals are received from the mobile transmitter 112. The secondmonitoring unit 116 further includes a processor circuit 121 forprocessing signals received by the second receiver 119 to produceprocessed signals and includes a transmitter 123 operable to transmitprocessed signals to the monitoring station 118.

The first and second monitoring units 114 and 116 may both be mobile,but it is contemplated that the first monitoring unit 114 will be partof a mobile wireless communication appliance 130 carried on or by theincarcerated individual and thus the beacon signal produced by themobile transmitter 112 which is worn by the individual is indicative ofthe presence of the first monitoring unit. This physical relationship isexploited as described below. The processor circuit 115 of the firstmonitoring unit is in communication with a processor of the mobilewireless communication appliance which in turn is in communication witha wireless transceiver thereof so that the processor can communicatewith the monitoring station and other devices, such as the signalingapparatus 101. Thus, the wireless transceiver of the mobile wirelesscommunication appliance itself may act as the transmitter 117 of thefirst monitoring unit 114.

The second monitoring unit 116 may be part of a stationary communicationappliance 132 and the processor circuit 121 and transmitter 123 thereofmay be operable to communicate with the monitoring station 118 and otherdevices such as the signaling apparatus 101 by a landline or may includea transceiver (not shown) to facilitate communications with themonitoring station and other devices such as the signaling apparatus 101by wireless methods.

The second monitoring unit 116 may be geographically located near adocking station 134 or may be part of a docking station operable torecharge the mobile wireless communication appliance, as will bedescribed in further detail below.

Referring to FIGS. 3 and 4, the signaling apparatus 101 includes adetector 120 operable to produce a proximity signal indicating proximityof the first monitoring unit 114 relative to the second monitoring unit116 and includes first and second transmitters 122 and 124. The firsttransmitter 122 is operable to cause the proximity signal to becommunicated to the first monitoring unit (114) and the processorcircuit 115 of the first monitoring unit is configured such that, whenthe proximity signal indicates the first monitoring unit is in proximityto the second monitoring unit 116 the first receiver 113 of the firstmonitoring unit is put into a low power mode in which signals are notreceived from the mobile transmitter 112. In general, the entire firstmonitoring unit may be placed in a low power mode.

When the proximity signal indicates that the first monitoring unit 114is not in proximity to the second monitoring unit 116 the first receiver113 and more generally the entire first monitoring unit, is set into areceive mode so that signals from the mobile transmitter 112 arereceived and processed by the first monitoring unit.

The second transmitter 124 is operable to cause the proximity signal tobe transmitted to the second monitoring unit 116 and the processorcircuit 121 of the second monitoring unit is configured such that inresponse to the proximity signal, the second monitoring unit receivesand processes signals from the mobile transmitter 112 when the proximitysignal indicates the first monitoring unit 114 is in proximity to thesecond monitoring unit and ceases receiving signals from the mobiletransmitter when the proximity signal indicates the first monitoringunit is not in proximity to the second monitoring unit. Ceasingreception of signals at the second monitoring unit may involve settingthe second receiver 119 into a non-receive mode.

The detector (120) is configured to cause the proximity signal toindicate the first monitoring unit 114 is in proximity to the secondmonitoring unit 116 when the first monitoring unit is coupled to adocking port of the docking station. Coupling to the docking port of thedocking station 134 may involve receiving the mobile wirelesscommunication appliance 130 in a receptacle 135 of the docking station,plugging the mobile wireless communication appliance 130 into a pigtailconnector of the docking station, or engaging a wireless link such as aBluetooth® link, or infrared link between the docking station and themobile wireless communication appliance. Each of these components mayact as a docking port to which the first monitoring unit may be coupled.

Referring to FIGS. 3 and 4, the detector 120 is configured to produce aproximity signal to indicate the first monitoring unit 114 is not inproximity to the second monitoring unit 116 when the first monitoringunit is uncoupled from the docking station 134. Thus, when the mobilewireless communication appliance 130 is away from the docking station134 the detector 120 causes the proximity signal to indicate same andthe first transmitter 122 transmits the proximity signal to the firstmonitoring unit 114 causing it to actively receive signals from themobile transmitter 112 and the second transmitter 124 transmits theproximity signal to the second monitoring unit 116 causing the secondmonitoring unit 116 to ignore signals from the mobile transmitter. Thus,when the first monitoring unit 114 is not in proximity to the secondmonitoring unit 116 the first monitoring unit 114 transmits processedsignals to the monitoring station 118 while the second monitoring unit116 does not.

When the mobile wireless communication appliance 130 is in the dockingstation 134 the detector 120 produces the proximity signal to indicatesame and the first transmitter 122 transmits the proximity signal to thefirst monitoring unit 114 causing it to enter the low power mode inwhich it does not receive signals from the mobile transmitter 112 andthe second transmitter 124 sends the proximity signal to the secondmonitoring unit 116 causing it to receive and process signals from themobile transmitter 112. Thus, when the first monitoring unit 114 is notin proximity with the second monitoring unit 116, the second monitoringunit 116 transmits processed signals to the monitoring station 118 whilethe first monitoring unit 114 does not.

Detecting whether or not the mobile wireless communication appliance 130is coupled to or uncoupled from the docking station 134 may be achievedusing a docking station detector 136 employing the flag or batterycurrent direction methods described above in connection with theembodiment shown in FIGS. 1 and 2, for example, or other detectionmethods may be employed.

As an alternative to direct detection of whether or not the mobilewireless communication appliance 130 is coupled to or uncoupled from thedocking station 134, detecting proximity of first and second monitoringunits 114 and 116 may be determined by measuring signal strength of thebeacon signal received from the mobile transmitter 112, since the mobiletransmitter 112 and the first monitoring unit are carried by the sameincarcerated individual. In this embodiment, the detector 120 comprisesa signal strength detector 138 operable to detect signal strength ofsignals received from the mobile transmitter 112 and to produce theproximity signal in response to measured signal strength.

Many off-the-shelf radio frequency receivers have outputs representing asignal strength value of a signal received and demodulated from a radiofrequency source, in addition to a signal output at which a demodulatedversion of the radio frequency signal is produced. Such a radiofrequency receiver may act as the signal strength detector 138 toproduce a signal representing signal strength. The signal strengthdetector 138 may further include a processor circuit 140 configured todetermine whether or not the signal representing signal strength meets afirst signal strength criterion. The signal strength detector 138 causesthe proximity signal to indicate the first monitoring unit 114 is inproximity to the second monitoring unit 116 when measured signalstrength of the beacon signal from the mobile transmitter 112 meets thefirst signal strength criterion. The signal strength detector 138 may beconfigured to determine the first signal strength criterion is met whenthe measured signal strength is above a first signal strength thresholdvalue.

Similarly, the signal strength detector 138 causes the proximity signalto indicate the first monitoring unit 114 is not in proximity to thesecond monitoring unit 116 when measured signal strength of the beaconsignal meets a second signal strength criterion. The signal strengthdetector 138 may determine the second signal strength criterion is metwhen the measured signal strength is below a second signal strengththreshold value. The first and second signal strength threshold valuesmay be the same or different, or may differ over time depending onoperating conditions. For example, the first and second signal strengththreshold values may be set to values that create a hysteresis effect,whereby setting the proximity signal to indicate the first monitoringunit 114 is in proximity to the second monitoring unit 116 occurs at adifferent signal strength when measured signal strength of the beaconsignal is increasing as compared to the measured signal strength of thebeacon signal at which the proximity signal is set to indicate the firstmonitoring unit 114 is not in proximity to the second monitoring unit116 when measured signal strength of the beacon signal is decreasing.

Thus, while the mobile wireless communication appliance 130 and hencethe mobile transmitter 112 is sufficiently far away from the dockingstation 134 the signal strength detector 138 detects a relatively weakbeacon signal or no signal from the mobile transmitter 112 and causesthe proximity signal to indicate the first monitoring unit 114 is not inproximity to the second monitoring unit 116. The first monitoring unit114 thus receives and processes signals from the mobile transmitter andtransmits these signals to the monitoring station 118.

As the mobile wireless communication appliance 130 and hence the mobiletransmitter 112 is brought nearer the signal strength detector 138, thebeacon signal strength measured by the detector increases. When thebeacon signal strength meets the first criterion, the signal strengthdetector 138 causes the proximity signal to indicate the firstmonitoring unit 114 is in proximity to the second monitoring unit 116and the first and second transmitters 122 and 124 transmit the proximitysignal indicating this to the first and second monitoring units 114 and116 respectively. In response, the first monitoring unit 114 enters itslow power mode and the second monitoring unit 116 begins receiving andprocessing the signals received from the mobile transmitter 112 andtransmits the processed signals to the monitoring station 118. Thus, thesecond monitoring unit 116 “takes over” receiving and processing fromthe first monitoring unit 114 when the first monitoring unit is broughtinto proximity with the second monitoring unit.

As a further alternative to detecting the presence of the mobilewireless communication appliance in the docking station, and tomeasuring signal strength, the detector 120 may include a positionsignal receiver 144 operable to receive position signals representing ageographical position of the first monitoring unit (114), and thedetector 120 produces the proximity signal in response to the positionsignals. The position signal receiver 144 may be of a type thatcomprises a global positioning system (GPS) signal receiver 146, or aLong Range Radio Navigation (LORAN) receiver 148, for example.Alternatively, the position signal receiver 144 may include a processorcircuit 150, which may communicate with an interface 152 to the wirelessnetwork or public switched telephone network. The position signalreceiver 144 may further include a message receiver 154 operable toreceive messages from a messaging system, the messages including datarepresenting position signals specifying the geographical position ofthe first monitoring unit 114. Effectively, a position signal producedby the wireless network by triangulation, may be formatted by thewireless network in a message compatible with the SMS message format orthe GPRS/CDMA 1X, CDPD, Mobitex, or 1XRTT data packet format, or anyother suitable format, for example.

In an embodiment employing the position signal receiver 144, regardlessof how the position signals are received at the apparatus, the detector120 is operable to cause the proximity signal to indicate the firstmonitoring unit 114 is in proximity to the reference position when theposition signals meet a first position criterion. The detector 120 maybe configured to determine the first position criterion is met when theposition signals indicate the first monitoring unit 114 is within afirst range of the reference position. For example, the detector 120 maybe configured to cause the proximity signal to indicate the apparatus 30is in proximity to the reference position when the position signalsindicate the apparatus is within 300 feet (100 m) of the referenceposition. The reference position may correspond to the geographicalposition of the second monitoring unit 116.

Similarly, the detector 120 may be operable to cause the proximitysignal to indicate the first monitoring unit 114 is not in proximity tothe reference position when the position signals meet a second positioncriterion and the detector may be configured to determine the secondposition criterion is met when the position signals indicate the firstmonitoring unit 114 is within a second range of the reference position.For example, the detector 120 may be configured to cause the proximitysignal to indicate the first monitoring unit is not in proximity to thereference position when the position signals indicate the firstmonitoring unit is more than 300 feet (100 m) from the referenceposition.

Regardless of how the proximity signal is produced, in one embodiment,the first transmitter 122 is operable to transmit a message representingthe proximity signal to the first monitoring unit 114. To facilitatethis, in one embodiment, the first transmitter 122 includes a messagingservice interface 156 operable to transmit a message to the firstmonitoring unit 114 through a messaging service. The messaging serviceinterface 156 may be operable to transmit a message compatible with theSMS message format or the GPRS/CDMA 1X, CDPD, Mobitex, or 1XRTT datapacket format, or any other suitable format, for example.

Similarly, in one embodiment the second transmitter 124 may be operableto transmit a message representing the proximity signal to the secondmonitoring unit and the second transmitter may include a messagingservice interface 158 operable to transmit a message to the secondmonitoring unit through a messaging service which may be the same or adifferent messaging service used by the first transmitter.

Referring to FIG. 5, in one embodiment the signaling apparatus shown inFIG. 4, and the monitoring unit 114 shown in FIG. 3 may be housed in acommon housing such as a modular battery unit 162 of the mobile wirelesscommunication appliance 130. In this embodiment, the modular batteryunit 162 includes a battery 161 operable to supply power to the mobilewireless communication appliance 130 and includes a bus interface 164 incommunication with a processor circuit 160 of the wireless communicationappliance. The signaling apparatus 101 described above is partlyimplemented by a processor circuit 168 operable to implement the firstand second transmitters 122 and 124 and includes the detector 120. Thedetector 120 may include any or a combination of the docking stationdetector 136, the signal strength detector 138 or the position signalreceiver 144. The processor circuit 168 may implement the processorcircuit 115 of the first monitoring unit 114 and may be connected to thefirst receiver 113 which is also housed in the modular battery unit 162.Simple modifications that would be apparent to those in the art may bemade to a program controlling the wireless communication applianceprocessor circuit 160 to facilitate communication between the processorcircuit 168 and the processor circuit 160 of the wireless communicationappliance to permit a radio transceiver 163 of the wirelesscommunication appliance to be used to transmit signals produced by theprocessor circuit 168 to the monitoring station 118 in response to thebeacon signal.

As will be appreciated the proximity signal may be produced by a varietyof methods as described above. The proximity signal is transmitteddirectly to the first receiver 113 to directly control it to place it inthe receive mode or the low power mode, depending on the state of theproximity signal. The processor circuit 168 acting in its capacity asthe second transmitter 124, also transmits a message representing theproximity signal to the wireless communication appliance processorcircuit 160 which causes the radio transceiver 163 to transmit themessage via any suitable messaging format such as those described aboveto the second monitoring unit 116 shown in FIG. 3. In response thereceiver 119 of the second monitoring unit 116, is placed in a receivemode or a non-receive mode depending on the state of the proximitysignal.

Referring to FIG. 6, in another embodiment, the signaling apparatus 101is housed in the docking station 134, in addition to the secondmonitoring unit 116. The detector 120 of the signaling apparatus 101 isin communication with the processor circuit 121 and may include thedocking station detector 136, the signal strength detector 138 or theposition signal receiver 144 described above. The processor circuit 121itself may perform some of the functions of the detector 120 includingdetermining whether criteria are met and producing the proximity signalaccordingly, as described above. The geographical position of thedocking station 134 may be taken as the reference position in settingthe criteria for setting the proximity signal active in response to theposition signals, for example.

In addition the processor circuit 121 is configured to implement thefirst and second transmitters 122 and 124 whereby the first transmitterincludes a message service interface 156 to facilitate transmitting amessage via the transmitter 123 to the first monitoring unit 114, themessage representing the proximity signal, for controlling the firstreceiver 113 of the first monitoring unit 114. In addition, theprocessor circuit 121 is configured to directly control the secondreceiver 119 to transmit the proximity signal thereto to cause it toenter the receive mode or non-receive mode, as appropriate, in responseto the proximity signal and thus acts as the second transmitter 124. Themessage may be in any of the formats described above, for example.

When the proximity signal is set active to indicate the first monitoringunit 114 is in proximity to the second monitoring unit in response towhichever detector type is used, the processor circuit 121 sends asignal to the second receiver 119 causing it to enter its receive modeand causes a message to be sent to the first monitoring unit 114 usingthe transmitter 123, to cause the first receiver 113 of the firstmonitoring unit 114 to enter the low power mode. When the proximitysignal is set inactive to indicate the first monitoring unit 114 is notin proximity to the second monitoring unit 116, the processor circuit121 sends a signal to the second receiver 119 causing it to enter itsnon-receive mode and causes a message to be sent to the first monitoringunit to place the first receiver 113 thereof into the receive mode. Inthis way the second receiver 119 in the docking station receives thebeacon signal from the mobile transmitter 112 when the first monitoringunit 114 is in proximity to the second monitoring unit 116 and the firstreceiver 113 in the first monitoring unit receives the beacon signalfrom the mobile transmitter when the first monitoring unit is not inproximity to the second monitoring unit.

While specific embodiments of the invention have been described andillustrated, such embodiments should be considered illustrative of theinvention only and not as limiting the invention as construed inaccordance with the accompanying claims.

1. A method for operating a monitoring unit operable to receive signalsfrom a mobile transmitter in an electronic location monitoring system,the method comprising: causing a receiver of the monitoring unit tochange a signal reception threshold thereof for receiving signals fromthe mobile transmitter, in response to proximity of the monitoring unitrelative to a reference position whereby said receiver rejects signalsreceived from the mobile transmitter that have a signal strength belowsaid signal reception threshold and receives and processes signalsreceived from the mobile transmitter that have a signal strength abovesaid signal reception threshold.
 2. The method of claim 1 whereinchanging said signal reception threshold comprises causing said receiverto decrease said signal reception threshold when said monitoring unit isin proximity to said reference position and causing said receiver toincrease said signal reception threshold when said monitoring unit isnot in proximity to said reference position.
 3. The method of claim 2further comprising detecting proximity of said monitoring unit relativeto said reference position.
 4. The method of claim 3 further comprisingproducing a proximity signal for reception by said receiver, in responseto detecting said change in proximity of said monitoring unit relativeto said reference position, whereby the receiver changes said signalreception threshold in response to said proximity signal.
 5. The methodof claim 4 further comprising causing said proximity signal to indicatesaid monitoring unit is in proximity to said reference position whensaid monitoring unit is coupled to a docking station.
 6. The method ofclaim 5 further comprising causing said proximity signal to indicatesaid monitoring unit is not in proximity to said reference position whensaid monitoring unit is uncoupled from a docking station.
 7. The methodof claim 4 further comprising receiving position signals at saidmonitoring unit, said position signals representing a geographicalposition of said monitoring unit, and producing said proximity signal inresponse to said position signals.
 8. The method of claim 7 causing saidproximity signal to indicate said monitoring unit is in proximity tosaid reference position when said position signals meet a first positioncriterion.
 9. The method of claim 8 wherein said first positioncriterion is met when said position signals indicate said monitoringunit is within a first range of said reference position.
 10. The methodof claim 9 causing said proximity signal to indicate said monitoringunit is not in proximity to said reference position when said positionsignals meet a second position criterion.
 11. The method of claim 10wherein said second position criterion is met when said position signalsindicate said monitoring unit is within a second range of said referenceposition.
 12. An apparatus operable to receive signals from a mobiletransmitter in an electronic location monitoring system, the apparatuscomprising: a proximity signal generator operable to generate aproximity signal in response to proximity of the apparatus relative to areference position; a receiver operable to receive signals from themobile transmitter, said receiver having a signal reception thresholdresponsive to said proximity signal, such that said receiver rejectssignals from the mobile transmitter that have a signal strength belowsaid signal reception threshold and receives signals from the mobiletransmitter that have a signal strength above said signal receptionthreshold; and a processor circuit operable to process signals receivedby said receiver to produce processed signals, said processor circuitbeing operable to communicate with a transmitter to cause thetransmitter to transmit said processed signals to a monitoring station.13. The apparatus of claim 12 wherein said receiver is operable todecrease said signal reception threshold when said proximity signalindicates the apparatus is in proximity to said reference position andwherein said receiver is operable to increase said signal receptionthreshold when said proximity signal indicates apparatus is not inproximity to said reference position.
 14. The apparatus of claim 13wherein said proximity signal generator comprises a detector operable todetect proximity of said apparatus to said reference position.
 15. Theapparatus of claim 14 wherein said detector is operable to detectcoupling of said apparatus to a docking station, and to cause saidproximity signal to indicate said apparatus is in proximity to saidreference position when said apparatus is coupled to the dockingstation.
 16. The apparatus of claim 15 wherein said detector is operableto detect uncoupling of said receiver from said docking station and tocause said proximity signal to indicate said apparatus is not inproximity to said reference position when said apparatus is uncoupledfrom the docking station.
 17. The apparatus of claim 14 wherein saiddetector comprises a position signal receiver operable to receiveposition signals representing a geographical position of said apparatus,said proximity signal generator being operable to produce said proximitysignal in response to said position signals.
 18. The apparatus of claim17 wherein said detector is operable to cause said proximity signal toindicate said apparatus is in proximity to said reference position whensaid position signals meet a first position criterion.
 19. The apparatusof claim 18 wherein said detector is configured to determine said firstposition criterion is met when said position signals indicate saidapparatus is within a first range of said reference position.
 20. Theapparatus of claim 19 wherein said detector is operable to cause saidproximity signal to indicate said apparatus is not in proximity to saidreference position when said position signals meet a second positioncriterion.
 21. The apparatus of claim 20 wherein said detector isconfigured to determine said second position criterion is met when saidposition signals indicate said apparatus is within a second range ofsaid reference position.
 22. The apparatus of claim 17 wherein saidposition signal receiver comprises a global positioning system (GPS)signal receiver.
 23. The apparatus of claim 17 wherein said positionsignal receiver includes a wireless receiver operable to receiveposition signals from a wireless communication network.
 24. Theapparatus of claim 17 wherein said position signal receiver comprises amessage receiver operable to receive messages from a messaging system.25. The apparatus of claim 24 wherein said message receiver is operableto receive messages from a Short Messaging System (SMS).
 26. Theapparatus of claim 12 further comprising a modular battery unit, saidapparatus being incorporated into said modular battery unit.
 27. Theapparatus of claim 26 further comprising a first mobile wirelesstransceiver powered by said modular battery unit and in communicationwith said apparatus such that said wireless transceiver is operable totransmit said processed signals to the monitoring station.
 28. Theapparatus of claim 12 wherein said processor circuit is configured toimplement a portion of said proximity signal generator.
 29. Theapparatus of claim 12 wherein said processor circuit is configured tocause said processed signals to include a representation of saidproximity signal.
 30. An apparatus operable to receive signals from amobile transmitter in an electronic location monitoring system, theapparatus comprising: means for generating a proximity signal inresponse to proximity of the apparatus relative to a reference position;means for receiving signals from the mobile transmitter, said means forreceiving having a signal reception threshold responsive to saidproximity signal, such that signals from the mobile transmitter thathave a signal strength below said signal reception threshold arerejected and such that signals from the mobile transmitter that have asignal strength above said signal reception threshold are received; andmeans for processing signals received by said receiver to produceprocessed signals, said means for processing being operable tocommunicate with a transmitter to cause the transmitter to transmit saidprocessed signals to a monitoring station.
 31. The apparatus of claim 30wherein said means for receiving is operable to decrease said signalreception threshold when said proximity signal indicates the apparatusis in proximity to said reference position and operable to increase saidsignal reception threshold when said proximity signal indicatesapparatus is not in proximity to said reference position.
 32. A methodfor switching monitoring units in an electronic location monitoringsystem comprising a mobile transmitter operable to transmit to at leastone of first and second monitoring units operable to communicate with acommon monitoring station, the method comprising: detecting proximity ofthe first monitoring unit relative to the second monitoring unit;producing a proximity signal in response to detecting proximity of thefirst monitoring unit relative to the second monitoring unit, forreception by the first monitoring unit to cause the first monitoringunit to enter a receive mode in which processing of signals receivedfrom the mobile transmitter by the first monitoring unit is permittedwhen the first monitoring unit is not in proximity to the secondmonitoring unit or a low power mode in which processing of signalsreceived from the mobile transmitter by the first monitoring unit isprevented when the first monitoring unit is in proximity to the secondmonitoring unit; and causing the second monitoring unit to receive andprocess signals from the mobile transmitter when said proximity signalindicates the first monitoring unit is in proximity to the secondmonitoring unit and causing the second monitoring unit to ceasereceiving signals from the mobile transmitter when said proximity signalindicates the first monitoring unit is not in proximity to the secondmonitoring unit.
 33. The method of claim 32 further comprising causingsaid proximity signal to indicate the first monitoring unit is inproximity to the second monitoring unit when the first monitoring unitis coupled to a docking station associated with the second monitoringunit.
 34. The method of claim 33 further comprising causing saidproximity signal to indicate the first monitoring unit is not inproximity to the second monitoring unit when the first monitoring unitis uncoupled from a docking station associated with the secondmonitoring unit.
 35. The method of claim 32 wherein detecting comprisesmeasuring, near the second monitoring unit, signal strength of signalsreceived from the mobile transmitter, and wherein said proximity signalis produced in response to measured signal strength.
 36. The method ofclaim 35 causing said proximity signal to indicate the first monitoringunit is in proximity to the second monitoring unit when measured signalstrength meets a first signal strength criterion.
 37. The method ofclaim 36 wherein said first signal strength criterion is met when saidmeasured signal strength is above a first signal strength thresholdvalue.
 38. The method of claim 37 causing said proximity signal toindicate the first monitoring unit is not in proximity to the secondmonitoring unit when measured signal strength meets a second signalstrength criterion.
 39. The method of claim 38 wherein said secondsignal strength criterion is met when said measured signal strength isbelow a second signal strength threshold value.
 40. The method of claim39 wherein said first and second signal strength threshold values arethe same.
 41. The method of claim 32 wherein detecting comprisesreceiving position signals at the first monitoring unit, said positionsignals representing a geographical position of the first monitoringunit, and producing said proximity signal in response to said positionsignals.
 42. The method of claim 41 causing said proximity signal toindicate the first monitoring unit is in proximity to the secondmonitoring unit when said position signals meet a first positioncriterion.
 43. The method of claim 42 wherein said first positioncriterion is met when said position signals indicate the firstmonitoring unit is within a first range of a reference position.
 44. Themethod of claim 43 causing said proximity signal to indicate the firstmonitoring unit is not in proximity to the second monitoring unit whensaid position signals meet a second position criterion.
 45. The methodof claim 44 wherein said second position criterion is met when saidposition signals indicate the first monitoring unit is within a secondrange of said reference position.
 46. The method of claim 45 whereinsaid reference position represents an approximate geographical positionof the second monitoring unit.
 47. The method of claim 32 whereinproducing said proximity signal comprises causing a message to betransmitted to the first monitoring unit.
 48. The method of claim 47wherein causing a message to be transmitted to the first monitoring unitcomprises causing a message to be transmitted to the first monitoringunit through a messaging service.
 49. The method of claim 48 whereincausing a message to be transmitted to the first monitoring unit througha messaging service comprises causing a Short Messaging Service (SMS)message to be transmitted to the first monitoring unit.
 50. The methodof claim 49 wherein said SMS message includes a power down command whenthe first monitoring unit is in proximity to the second monitoring unitand wherein said SMS message includes a power up command when the firstmonitoring unit is not in proximity to the second monitoring unit. 51.An apparatus for switching monitoring units in an electronic locationmonitoring system comprising a mobile transmitter operable to transmitto at least one of first and second monitoring units operable to processsignals received from the mobile transmitter and transmit processedsignals to a monitoring station, the apparatus comprising: a detectoroperable to produce a proximity signal indicating proximity of the firstmonitoring unit relative to the second monitoring unit; a firsttransmitter operable to cause said proximity signal to be communicatedto the first monitoring unit such that, when the proximity signalindicates the first monitoring unit is in proximity to the secondmonitoring unit the first monitoring unit enters a low power mode inwhich signals are not received from the mobile transmitter by the firstmonitoring unit and when the proximity signal indicates that the firstmonitoring unit is not in proximity to the second monitoring unit thefirst monitoring unit enters a receive mode in which signals from themobile transmitter are received and processed by the first monitoringunit; and a second transmitter operable to cause said proximity signalto be transmitted to the second monitoring unit such that in response tothe proximity signal the second monitoring unit receives and processessignals from the mobile transmitter when said proximity signal indicatesthe first monitoring unit is in proximity to the second monitoring unitand ceases receiving signals from the mobile transmitter when saidproximity signal indicates the first monitoring unit is not in proximityto the second monitoring unit.
 52. The apparatus of claim 51 whereinsaid detector is operable to cause said proximity signal to indicate thefirst monitoring unit is in proximity to the second monitoring unit whenthe first monitoring unit is coupled to a docking station.
 53. Theapparatus of claim 52 wherein said detector is operable to cause saidproximity signal to indicate the first monitoring unit is not inproximity to the second monitoring unit when the first monitoring unitis uncoupled from a docking station.
 54. The apparatus of claim 51wherein said detector comprises a signal strength detector operable todetect signal strength of signals received from the mobile transmitter,and wherein said detector produces said proximity signal in response tomeasured signal strength.
 55. The apparatus of claim 54 wherein saiddetector causes said proximity signal to indicate the first monitoringunit is in proximity to the second monitoring unit when measured signalstrength meets a first signal strength criterion.
 56. The apparatus ofclaim 55 wherein said detector determines said first signal strengthcriterion is met when said measured signal strength is above a firstsignal strength threshold value.
 57. The apparatus of claim 56 whereinsaid detector causes said proximity signal to indicate the firstmonitoring unit is not in proximity to the second monitoring unit whenmeasured signal strength meets a second signal strength criterion. 58.The apparatus of claim 57 wherein said detector determines said secondsignal strength criterion is met when said measured signal strength isbelow a second signal strength threshold value.
 59. The apparatus ofclaim 58 wherein said first and second signal strength threshold valuesare the same.
 60. The apparatus of claim 51 wherein said detectorcomprises a position signal receiver operable to receive positionsignals representing a geographical position of the first monitoringunit, and wherein said detector produces said proximity signal inresponse to said position signals.
 61. The apparatus of claim 60 whereinsaid detector causes said proximity signal to indicate the firstmonitoring unit is in proximity to the second monitoring unit when saidposition signals meet a first position criterion.
 62. The apparatus ofclaim 61 wherein said detector determines said first position criterionis met when said position signals indicate the first monitoring unit iswithin a first range of a reference position.
 63. The apparatus of claim62 wherein said detector causes said proximity signal to indicate thefirst monitoring unit is not in proximity to the second monitoring unitwhen said position signals meet a second position criterion.
 64. Theapparatus of claim 63 wherein said detector determines said secondposition criterion is met when said position signals indicate the firstmonitoring unit is within a second range of said reference position. 65.The apparatus of claim 64 wherein said reference position represents anapproximate geographical position of the second monitoring unit.
 66. Theapparatus of claim 51 wherein said first transmitter is operable totransmit a message representing said proximity signal to the firstmonitoring unit.
 67. The apparatus of claim 66 further comprising afirst messaging service interface operable to transmit a message to thefirst monitoring unit through a messaging service.
 68. The apparatus ofclaim 67 wherein said first messaging service interface is operable totransmit a message compatible with a Short Messaging Service (SMS)format to be transmitted to the first monitoring unit.
 69. The apparatusof claim 51 wherein said second transmitter is operable to transmit amessage representing said proximity signal to the second monitoringunit.
 70. The apparatus of claim 69 further comprising a secondmessaging service interface operable to transmit a message to the secondmonitoring unit through a messaging service.
 71. The apparatus of claim70 wherein said second messaging service interface is operable totransmit a message compatible with a Short Messaging Service (SMS)format to be transmitted to the second monitoring unit.
 72. Theapparatus of claim 51 further comprising a modular battery unit,operable to power the first monitoring unit, said detector, said firsttransmitter and said second transmitter being incorporated into saidmodular battery unit.
 73. The apparatus of claim 51 further comprising ahousing, wherein said detector, said first transmitter, said secondtransmitter and the second monitoring unit are housed in said housing.74. The apparatus of claim 73 wherein said housing has a docking stationto facilitate docking of the first monitoring unit.
 75. An apparatus foruse in an electronic location monitoring system, the apparatuscomprising: a first monitoring unit comprising: a first receiveroperable to receive signals from a mobile transmitter on a person to bemonitored, the first receiver having a low power mode in which signalsare not received from the mobile transmitter and a receive mode in whichsignals from the mobile transmitter are received; a processor circuitfor processing signals received by said first receiver to produceprocessed signals; and a transmitter operable to transmit said processedsignals to a monitoring station; and a signaling unit comprising: adetector operable to produce a proximity signal indicating proximity ofthe first monitoring unit relative to a second monitoring unit operableto receive, process and transmit signals to the monitoring station; saidfirst receiver being responsive to said proximity signal such that, whenthe proximity signal indicates the first monitoring unit is in proximityto the second monitoring unit the first receiver is placed in the lowpower mode and when the proximity signal indicates that the firstmonitoring unit is not in proximity to the second monitoring unit thefirst receiver enters the receive mode; a proximity signal transmitteroperable to cause said proximity signal to be transmitted to the secondmonitoring unit, whereby in response to the proximity signal the secondmonitoring unit receives and processes signals from the mobiletransmitter when said proximity signal indicates said first monitoringunit is in proximity to the second monitoring unit and ceases receivingsignals from the mobile transmitter when said proximity signal indicatessaid first monitoring unit is not in proximity to the second monitoringunit, whereby said first monitoring unit receives and processes signalsfrom the mobile transmitter and the second monitoring unit does notreceive and process signals from the mobile transmitter when said firstmonitoring unit is not in proximity to the second monitoring unit andwhereby the second monitoring unit receives and processes signals fromthe mobile transmitter and said first monitoring unit does not receiveand process signals from the mobile transmitter when said firstmonitoring unit is in proximity to the second monitoring unit.
 76. Theapparatus of claim 75 further comprising a modular battery unit operableto power said first monitoring unit and said signaling unit and whereinsaid first monitoring unit and said signaling unit are housed in saidmodular battery unit.
 77. The apparatus of claim 75 further comprising aportable wireless communication appliance, said apparatus beingincorporated into said portable wireless communication appliance. 78.The apparatus of claim 77 wherein said portable wireless communicationappliance includes a cellular telephone.
 79. The apparatus of claim 75wherein said detector is operable to cause said proximity signal toindicate said first monitoring unit is in proximity to the secondmonitoring unit when said first monitoring unit is coupled to a dockingstation.
 80. The apparatus of claim 79 wherein said detector is operableto cause said proximity signal to indicate said first monitoring unit isnot in proximity to the second monitoring unit when said firstmonitoring unit is uncoupled from a docking station.
 81. The apparatusof claim 76 wherein said detector comprises a signal strength detectoroperable to detect signal strength of signals received from said mobiletransmitter, and wherein said detector produces said proximity signal inresponse to measured signal strength.
 82. The apparatus of claim 81wherein said detector causes said proximity signal to indicate saidfirst monitoring unit is in proximity to the second monitoring unit whenmeasured signal strength meets a first signal strength criterion. 83.The apparatus of claim 82 wherein said detector determines said firstsignal strength criterion is met when said measured signal strength isabove a first signal strength threshold value.
 84. The apparatus ofclaim 83 wherein said detector causes said proximity signal to indicatesaid first monitoring unit is not in proximity to the second monitoringunit when measured signal strength meets a second signal strengthcriterion.
 85. The apparatus of claim 84 wherein said detectordetermines said second signal strength criterion is met when saidmeasured signal strength is below a second signal strength thresholdvalue.
 86. The apparatus of claim 85 wherein said first and secondsignal strength threshold values are the same.
 87. The apparatus ofclaim 75 wherein said detector comprises a position signal receiveroperable to receive position signals representing a geographicalposition of said first monitoring unit, and wherein said detectorproduces said proximity signal in response to said position signals. 88.The apparatus of claim 87 wherein said detector causes said proximitysignal to indicate said first monitoring unit is in proximity to thesecond monitoring unit when said position signals meet a first positioncriterion.
 89. The apparatus of claim 88 wherein said detectordetermines said first position criterion is met when said positionsignals indicate said first monitoring unit is within a first range of areference position.
 90. The apparatus of claim 89 wherein said detectorcauses said proximity signal to indicate said first monitoring unit isnot in proximity to the second monitoring unit when said positionsignals meet a second position criterion.
 91. The apparatus of claim 90wherein said detector determines said second position criterion is metwhen said position signals indicate said first monitoring unit is withina second range of said reference position.
 92. The apparatus of claim 91wherein said reference position represents an approximate geographicalposition of the second monitoring unit.
 93. The apparatus of claim 75wherein said proximity signal transmitter is operable to transmit amessage representing said proximity signal to the second monitoringunit.
 94. The apparatus of claim 93 further comprising a messagingservice interface operable to transmit a message to the secondmonitoring unit through a messaging service.
 95. The apparatus of claim94 wherein said messaging service interface is operable to transmit amessage compatible with a Short Messaging Service (SMS) format to betransmitted to the second monitoring unit.
 96. A docking apparatus for afirst monitoring unit in an electronic location monitoring system inwhich the first monitoring unit has a first receiver operable to receivesignals from a mobile transmitter on a person to be monitored andwherein said first monitoring unit is operable to process said signalsto produce processed signals and to transmit said processed signals to amonitoring station, the docking apparatus comprising: a secondmonitoring unit comprising: a second receiver operable to receivesignals from the mobile transmitter, the second receiver having anon-receive mode in which signals are not received from the mobiletransmitter and a receive mode in which signals from the mobiletransmitter are received; a processor circuit for processing signalsreceived by said second receiver to produce processed signals; and atransmitter operable to transmit said processed signals to a monitoringstation; and a signaling unit comprising: a detector operable to producea proximity signal indicating proximity of the first monitoring unitrelative to the second monitoring unit; said second receiver beingresponsive to said proximity signal such that, when the proximity signalindicates the first monitoring unit is in proximity to the secondmonitoring unit the second receiver is placed in the receive mode andwhen the proximity signal indicates that the first monitoring unit isnot in proximity to the second receiver is placed in the non-receivemode; a proximity signal transmitter operable to cause said proximitysignal to be transmitted to the first monitoring unit, whereby inresponse to the proximity signal the first monitoring unit receives andprocesses signals from the mobile transmitter when said proximity signalindicates said first monitoring unit is not in proximity to the secondmonitoring unit and ceases receiving signals from the mobile transmitterwhen said proximity signal indicates said first monitoring unit is inproximity to the second monitoring unit, whereby said first monitoringunit receives and processes signals from the mobile transmitter and thesecond monitoring unit does not receive and process signals from themobile transmitter when said first monitoring unit is not in proximityto the second monitoring unit and whereby the second monitoring unitreceives and processes signals from the mobile transmitter and saidfirst monitoring unit does not receive and process signals from themobile transmitter when said first monitoring unit is in proximity tothe second monitoring unit.
 97. The apparatus of claim 96 wherein saiddocking station apparatus includes a docking port to which the firstmonitoring unit may be coupled.
 98. The apparatus of claim 97 whereinsaid detector is operable to cause said proximity signal to indicate thefirst monitoring unit is in proximity to said second monitoring unitwhen the first monitoring unit is coupled to said docking station. 99.The apparatus of claim 97 wherein said detector is operable to causesaid proximity signal to indicate the first monitoring unit is not inproximity to said second monitoring unit when the first monitoring unitis uncoupled from said docking station.
 100. The apparatus of claim 96wherein said detector comprises a signal strength detector operable todetect signal strength of signals received from said mobile transmitter,and wherein said detector produces said proximity signal in response tomeasured signal strength.
 101. The apparatus of claim 100 wherein saiddetector causes said proximity signal to indicate the first monitoringunit is in proximity to said second monitoring unit when measured signalstrength meets a first signal strength criterion.
 102. The apparatus ofclaim 101 wherein said detector determines said first signal strengthcriterion is met when said measured signal strength is above a firstsignal strength threshold value.
 103. The apparatus of claim 102 whereinsaid detector causes said proximity signal to indicate the firstmonitoring unit is not in proximity to said second monitoring unit whenmeasured signal strength meets a second signal strength criterion. 104.The apparatus of claim 103 wherein said detector determines said secondsignal strength criterion is met when said measured signal strength isbelow a second signal strength threshold value.
 105. The apparatus ofclaim 104 wherein said first and second signal strength threshold valuesare the same.
 106. The apparatus of claim 63 wherein said detectorcomprises a position signal receiver operable to receive positionsignals representing a geographical position of the first monitoringunit, and wherein said detector produces said proximity signal inresponse to said position signals.
 107. The apparatus of claim 106wherein said detector causes said proximity signal to indicate the firstmonitoring unit is in proximity to said second monitoring unit when saidposition signals meet a first position criterion.
 108. The apparatus ofclaim 107 wherein said detector determines said first position criterionis met when said position signals indicate the first monitoring unit iswithin a first range of a reference position.
 109. The apparatus ofclaim 108 wherein said detector causes said proximity signal to indicatethe first monitoring unit is not in proximity to said second monitoringunit when said position signals meet a second position criterion. 110.The apparatus of claim 109 wherein said detector determines said secondposition criterion is met when said position signals indicate the firstmonitoring unit is within a second range of said reference position.111. The apparatus of claim 110 wherein said reference positionrepresents an approximate geographical position of said secondmonitoring unit.
 112. The apparatus of claim 96 wherein the firsttransmitter is operable to transmit a message representing saidproximity signal to the first monitoring unit.
 113. The apparatus ofclaim 112 further comprising a messaging service interface operable totransmit a message to the first monitoring unit through a messagingservice.
 114. The apparatus of claim 113 wherein said messaging serviceinterface is operable to transmit a message compatible with a ShortMessaging Service (SMS) format to be transmitted to the first monitoringunit.